In a television system, the composite signal that is modulated onto a radio frequency carrier wave is comprised of video signals obtained by scanning the scene to be televised along successive parallel lines of a first field and thenalong parallel lines of a second field that are interleaved with the lines of a first field to create what is called a frame. The lines of the respective fields of successive frames occur in the same place so that the video signals along the lines owuld be identical if there were no changes in the scene. Line synchronizing signals associated with each line and field synchronizing signals associated with each field are included for the purpose of causing the beam of a cathode ray tube of a monochrome receiver or the beams of a cathode ray tube of a color television receiver to scan their phosphorized inner faces in the same pattern of lines and fields.
In monochrome T.V. systems, the video signal is a luminance signal within a frequency band at six megacycles, and, as is well known, the fineness of detail that can be represented increases with frequency. In PAL (phase alternate line) color T.V. systems, the color information is conveyed by a color subcarrier that is superimposed onto the luminance signal at about 4.4 megacycles. Although the frequency of the subcarrier with respect to the line scanning frequency of 15.625 KHz is chosen such that it comes up out of phase after a given number of frames so as to tend to cancel its effect on luminance and therefore be less visible on monochrome receivers as well as color receivers, the subcarrier is still visible on receivers producing luminance signals within the frequency band occupied by the color subcarrier and its sidebands. One way of avoiding this problem is to gradually lower the response to the luminance signal to a very low value at the frequencies of the subcarrier components, but this reduces the fineness of the detail that can be displayed.
In order to permit higher frequencies of the luminance signal to be used and therefore to provide more detail in the image, comb filters have been employed to remove the energy introduced into the luminance signal by the presence of the color subcarrier. They are called comb filters because subcarrier energy occurs at uniformly spaced frequencies.
In the PAL color television system, the color subcarrier generated by combining the amplitude modulations of its 0.degree. and 90.degree. phases with R-Y and B-Y color difference signals respectively is added to a Y luminance signal.
The ratio between the frequency of the color subcarrier and the frequency at which the lines are scanned is such that 283.75 cycles occur during a line scanning interval so that the carrier shifts by 90.degree. with respect to each successive line. Since each quadrature phase represents a different color difference signal, it is customary to refer to them as pixels distributed along a line. Assuming sampling at four times the subcarrier frequency, there are 1135 pixels in each line, each of which represents a quadrature phase. The table below indicates the phase shifts for different numbers of lines, H.
1 pixel=90.degree. PA1 1 H=90.degree. PA1 2 H=180.degree. PA1 312 H=180.degree. PA1 1 frame=625 H=90.degree. PA1 1 frame+1 pixel=180.degree.
As a matter of terminology, combs that use data from only one field are referred to as 2D combs and combs that use data from two or more fields are called 3D combs.
Decoders that change their mode of operation in response to the video signal are known as adaptive decoders. Whereas they can obtain excellent results, there are situations where they flip back and forth between modes so as to produce very poor results. For this reason, non-adaptive decoders that use the same mode of operation regardless of the video signal are preferable even though their results may not be as good as an adaptive filter at its best.